Concept: A force is a push or a pull on an object, and it can come from direct contact (like pushing a door) or from a distance (like gravity pulling you down). Forces can make objects start moving, stop moving, speed up, slow down, or change direction.
In this lesson, we'll learn how forces can balance each other out or become unbalanced and cause changes in an object's motion.
2. Check Understanding: What Is a Force?
Which of the following best describes a force?
Which statement is most accurate?
3. What Is a Resultant Force?
The resultant force (or net force) is the overall effect of all forces acting on an object. If you have multiple forces in different directions, you can add or subtract them to find one single, overall force.
For example, if two people push on opposite sides of a box with the same strength, the forces can cancel out. If one person pushes harder, the box moves in that direction. The resultant force tells us which way (and how strongly) an object will move or accelerate.
4. Practice: Identifying Resultant Forces
Reminder: If two forces act in opposite directions, the resultant force is the difference between them. If they act in the same direction, the resultant is their sum.
Question 1: Two people pull on a rope in opposite directions. Person A pulls with 50 N, and Person B pulls with 30 N. What is the resultant force?
Question 2: A rocket engine pushes upwards with 5000 N, while gravity pulls it down with 4000 N. What is the resultant force acting on the rocket?
Question 3: A car is pushed forward by an engine force of 2000 N while air resistance and friction total 800 N in the opposite direction. What is the resultant force?
Question 4: Two people push a box in the same direction, each with 25 N. No other forces act horizontally. What is the resultant force on the box (in that horizontal direction)?
Question 5: A block is pushed left by 10 N and pushed right by 6 N at the same time. What is the resultant force?
5. Balanced Forces
Balanced forces occur when all the forces acting on an object cancel each other out, giving a resultant force of zero.
Examples:
A book resting on a table: Gravity pulls down, the table pushes up equally.
A person standing still on the floor: Weight (down) is balanced by the floor’s push (up).
When forces are balanced, the object will either stay still (if it was already still) or keep moving at a constant speed (if it was already moving). There is no change in motion.
6. Practice: Balanced Forces
Key Idea: If the resultant force is zero, there's no acceleration. The object remains at rest or continues at constant velocity.
Question 1: A book lying on a table is not moving. Which statement explains why?
Question 2: If an object moves at a constant speed in a straight line, what can we say about the forces on it?
Question 3: A ball is floating in water, neither sinking nor rising. What does this tell you about the forces on the ball?
Question 4: A car travels along a straight road at a steady 60 km/h. Which statement is correct?
7. Unbalanced Forces
Unbalanced forces occur when the resultant force is not zero. In other words, one or more forces acting on the object are stronger in one direction than in another.
This causes a change in the object's motion: it may start moving, stop moving, speed up, slow down, or change direction.
8. Practice: Unbalanced Forces
Key Idea: If the forces do not cancel out, the object accelerates or decelerates depending on the direction of the net force.
Question 1: If the forward force on a car is greater than friction, what happens to the car?
Question 2: If friction is suddenly greater than the forward push, what happens to the car’s motion?
Question 3: A cyclist is riding east. A strong wind blows from the north. What will happen to the cyclist’s direction of travel?
Question 4: An object is already moving west. If an unbalanced force acts east, how does the object's motion change?
9. How Unbalanced Forces Change Speed or Direction
When forces are unbalanced, the object accelerates (speeding up, slowing down, or turning). The direction of the resultant force determines which way the object accelerates.
Speeding Up: Net force in the same direction as motion.
Slowing Down: Net force opposite to the direction of motion.
Changing Direction: Net force acts at an angle to the direction of motion.
In more advanced physics, this is explained by Newton’s Second Law: \( F = ma \), where force causes acceleration proportional to the mass of the object.
10. Practice: Effects of Unbalanced Forces
Tip: Think about which direction the net force acts compared to the object's motion.
Question 1: A cyclist is moving north at a steady speed. A strong gust of wind blows from the west. What happens to the cyclist’s motion?
Question 2: A car travels east. Suddenly the driver brakes. Which statement describes the forces?
Question 3: A runner moving south suddenly experiences a net force pushing them west. How does their path change?
Question 4: An object moves west. You apply a larger force to the east than any other forces. What will happen?
Practical Data: Observing Forces
Record any practical data you have from demonstrations or experiments with forces (e.g., measuring pull forces with a Newton meter, or friction on different surfaces). Enter your data below:
Scenario / Object
Force 1 (N)
Force 2 (N)
Resultant? (N)
Check
Conclusion
Based on your practical observations, describe when forces were balanced and when they were unbalanced. How did you know?